Chromatography is a widely used technique for separating organic and inorganic compounds and substances, e.g., amino acids, proteins, nucleic acids, hydrocarbons, and carbohydrates. Effective chromatography is influenced mainly by efficiency and selectivity of a chromatographic medium with respect to specific compounds of interest. Major improvements in column efficiency can be obtained by using more uniform particle size, or in the case of high performance liquid chromatography (HPLC), decreasing particle size of the packing material. For example in the past 25 years, typical particle size for HPLC has decreased from the early 10 μm (micrometer or micron) to 1.8 μm, which has resulted in greatly increased column efficiency. However, reduction in particle sizes results in a disadvantage which is a requirement for higher operating pressures and a need for more sophisticated design and expensive instrumentation such as ultra-high pressure liquid chromatography (UHPLC) systems. See Skoog et al. 1998 Principles of Instrumental Analysis (fifth edition) Brooks Cole publishing p. 725-743; and Skoog et al. 2006 Principles of Instrumental Analysis (sixth edition) Brooks Cole publishing chapter 28.
A more effective approach for improving compound resolution is to alter the column selectivity of the chromatographic medium with respect to specific compounds. The packing material for liquid chromatography is commonly based on porous silica particles. The silanol functional groups on the silica surface can be modified by conjugating with a variety of alkyl and other functional groups to change the affinity and selectivity of the packing materials to different compounds. For example, attaching a C18 alkyl chain on the silanol group creates so called reversed-phase liquid chromatographic media that are widely used for separating polar organic compounds. Other ways of modifying the resolution properties of silica gel include physically attaching compounds or ions onto the surface of porous silica particles. For example, silver ion (Ag+) has been attached by a process referred to as impregnation onto the silica surface by immersing the silica gel in an aqueous solution of silver nitrate (AgNO3). Upon evaporating of water from the solution, the silver ion is deposited and electrostatically attached to the surface of silica gel on the silanol functional group. Because Ag+ ions strongly interact with double bonds in organic compounds, silver impregnated silica gel is widely used for resolving compounds with different degrees of unsaturation (i.e., different numbers of double bonds or triple bonds) by a process referred to as argentation chromatography. Argentation chromatography has been used for chromatographic separation for more than five decades and is considered a critical technique for separating carbon-carbon double bond containing compounds especially lipids.
Problems with argentation chromatography include instability of impregnated Ag+ ions, which are reduced to Ag0 upon exposure to light within minutes, and instability of the medium, as the ions are merely electrostatically affiliated with the silanol functional group of the silica gel and are easily washed out. Conventional argentation chromatography is generally performed in darkness, which is difficult to achieve in the laboratory, and the medium must be freshly prepared (either by the researchers themselves or by commercial providers) and stored in light blocking containers and under anhydrous conditions. Because a certain level of light exposure is unavoidable, even the most carefully prepared and light-shielded chromatographic column can be reproducibly used only once and must be discarded after a single separation, leading to increased expense, inconvenience, and irreproducible results. Hence current argentation chromatography is time consuming and expensive, particularly for large scale projects. Furthermore, because Ag+ ions are attached to the silica gel surface only electrostatically, during a chromatographic elution the Ag+ ions are mobile during interaction with double bond-containing organic compounds. The resulting partially mobile affinity material of an intended stationary phase support medium causes peak tailing and severe reduction in chromatographic resolution. In addition, Ag+ ions continuously bleed into the liquid eluants, which contaminates detection devices, and is particularly destructive to mass spectrometers, making HPLC mass spectrometry or HPLC-MS (which is an increasingly common technique for analyzing polar organic compounds) applications practically impossible to perform. These problems have served as roadblocks to further development of methods of separation that involve argentation chromatographic media.
Silver ion chromatography in spite of these shortcomings remains in use. There are currently no chromatographic media that specifically target dense electron clouds (such as triple bonds and double bonds) in the molecules and separate these molecules. Improved silver chromatographic materials are needed for medical, industrial and pharmaceutical applications.